The invention relates to a drive device for vehicles and more especially to a drive device for a wheeled vehicle suitable for use on very rough terrain comprising a drive prime mover coupled with a power splitting hydrostatic-mechanical transmission consisting of a planetary differential drive with at least four shafts, having two groups of planet wheels, two sun wheels, a web and an annulus, and furthermore comprising two main shafts forming the input and output and connected with different shafts of the planetary differential transmission, at least two variable displacement hydrostatic machines which are respectively connected in at least one operational range with a respective shaft of the planetary differential transmission and are adapted to alternately operate as a pump and as a motor, at least one first hydrostatic machine, when the second hydrostatic machine is at least substantially stationary, being able to be switched over from the input main shaft to the sun wheel, not on the drive side, on transition from one to the other operational range by means of clutch means and during traction operation switches over from the motor to the pump function.
A drive device with such features is for instance described in the U.S. Pat. No. 4,313,351. Such vehicle drive systems, designed for instance for city omnibus services, comprises a power splitting hydromechanical transmission with a planetary differential transmission and two hydrostatic machines in the form of adjustable hydrostatic devices able to be used both as a motor and as pump to transmit power, which is the product of the independent maximum permissible values of the parameters defining the power, of the same order of the drive prime mover. This drive system between the drive prime mover and the axle drive train makes possible an expansion of the conversion range or power ratio to a factor of the order of 6. The increase in torque is quite sufficient in the case of omnibuses or regular commercial vehicles.
In the case of drive trains which have to have wide range of conversion, as for instance wheeled vehicles able to be driven on extremely rough terrain, such a drive device has so far not been found satisfactory, because the said increase in the conversion range (by a factor of 6) is still not adequate for starting and at low speeds on terrain which is hard to negotiate. To be adequate for such an application the increase in torque between the drive engine and the axle drive train would have to be by a factor of the order of 25 at full engine power.